The invention relates to an ink jet writing head comprising a pressurized ink chamber provided with at least one ink nozzle that is closed by a mobile valve body and a piezoelectric valve actuating device that is electrically connected to a control device and drives the valve body with reciprocating motions.
Such ink jet writing heads are used to print letters and numbers on surfaces that move past. Usually an ink chamber accommodates a plurality of ink nozzles that the control actuates selectively to briefly open the respective actuated nozzle. Due to the pressure in the ink chamber a brief ink jet is pushed out of the ink nozzle and creates a point on the print surface.
In a customary ink jet writing head (EP 0 445 137) the valve actuating elements are hinged magnets whose anchors are connected to the respective valve body in a driving manner. Here, the mass substantially formed by the anchor of the hinged magnet must be accelerated and decelerated with each valve actuating process.
The energy required for this process limits the possible working speed of the valve actuating device.
In a known ink jet writing head as described in the introduction (DE 1 952 880 A) it is proposed to use piezoelectric valve actuating elements to increase the working speed. However, such ink jet writing heads do not meet the practical demands at higher writing speeds.
Therefore, it is desirable to provide an ink jet writing head that can be used trouble-free at speeds required for practical applications.
According to an aspect of the present invention, a valve body is held by a closing spring in the closing position thereof and is connected to a connecting rod in a driving manner, said connecting rod preferably extending from the ink chamber in a sealed and mobile manner, in that at least one curved spring strip engages with the connecting rod, the end of said spring strip being supported opposite the ink chamber, in that the connecting rod is subjected to the tractional pre-stress of a tension spring and in that at least one piezoelectric element is applied to the spring strip, said piezoelectric element being controlled by the control device in such a way as to perform expansion or contraction movements.
Preferably plane piezoelectric elements are used, with piezoelectric elements on foil basis being especially suitable.
It is also possible to use piezoelectric elements on the basis of fibers or strips, for example commercially available MFC piezoelectric elements (macro-fiber composite piezoelectric element) known from WO 01/33648 A1.
High-performance piezoelectric elements in the form of plane piezoelectric elements are available, however due to their design, they are not readily useable to actuate valves on ink jet writing heads. However, the connection with a least one curved spring strip on which a plane piezoelectric element is applied on one side provides a valve actuating device that provides sufficient performance and working speed with very limited space requirements and thus can be used together with an ink jet writing head. Above all, operability of the valve actuating device is preserved even with high use frequency over long periods of time.
Preferably a pre-stressed piezoelectric element is applied to the inside of the curve of the spring strip that is energized to perform tractile movements only. It is also possible to apply a piezoelectric element on the outside of the curve of the spring strip that can be energized to perform contraction movements. It is especially advantageous to apply piezoelectric elements on both sides of the spring strip. This results in an especially high actuating force.
According to a preferred embodiment of the invention, only one curved spring strip to which one piezoelectric element is applied engages with the end of the connecting rod that is located at a distance from the valve. This means the masses that are to be moved are relatively small so that it is possible to achieve a high actual frequency.
Here it is advantageous to guide the end of the spring strip or the connecting rod that is located at a distance from the valve in a mobile manner in longitudinal direction of the connecting rod.
It also is possible to have an embodiment, wherein two spring strips curved in opposite direction engage with the end of the connecting rod located at a distance from the valve, said spring strip being supported on the valve-side end thereof, opposite the ink chamber, and wherein the piezoelectric elements of the two spring strips can be actuated together by means of the control device in such a way as to perform the movements. This symmetric design ensures a symmetric application of force on the connecting rod without the presence of a guide device.
With its end located at a distance from the valve, the connecting rod can extend through a bore of an upper tensioning block and can comprise an adjusting nut on one thread section that allows adjusting the pre-stress. On the valve side, the connecting rod extends through a bore of a lower tensioning block in which the end of the spring strip or the ends of the two spring strips, respectively, is/are accommodated.
According to a first embodiment of the invention the connecting rod is connected directly to the valve body. This results in a direct force translation from the connecting rod to the valve body. This embodiment is especially suitable if there is only one single ink nozzle or if there is sufficient distance between a plurality of ink nozzles. It also is possible for the connecting rod to be connected in a driving manner via an actuating lever with a valve connecting rod that is connected to the valve closing body. This embodiment allows for a staggered arrangement of the valve actuating devices on both sides of the ink chamber so that it is possible to reach ink nozzles that are arranged in relatively close proximity to one another.